The present invention relates to power take off arrangements for motor vehicles.
“Power take off arrangement” as used herein refers to arrangements for transmitting rotary drive from the engine (or other rotary driver, such as an electric motor) of a motor vehicle to some point of power usage other than the vehicle's driven wheels. The phrase will be abbreviated to “PTO”. Agricultural vehicles such as tractors are conventionally provided with a PTO for connection to power driven attachments such as harvesters, grass cutters, driven-wheel trailers and so on. The PTO typically comprises a drive shaft coupled to the engine through some form of transmission and provided with means for forming a releasable coupling to a shaft of the attachment. PTOs are not solely used in agricultural vehicles. Military vehicles also use PTOs for driving driven-wheel trailers and other implements, and there are other fields in which PTOs have actual or potential applications.
Conventional agricultural PTOs are typically intended to run at fixed speed. A fixed ratio gearbox, typically separate from the vehicle's transmission, provides one or a set of drive ratios and the vehicle's speed-governed diesel engine is set to a fixed speed to provide the required PTO output speed. A clutch is needed between the engine and the PTO to de-couple the two when drive is not required at the PTO.
This conventional PTO technology suffers from several drawbacks. One is encountered upon “launch”—i.e. when applying drive to the PTO output to accelerate it from a standstill to the required operating speed. The driven implement often possesses a large moment of inertia, which needs to be accelerated over an appreciable period of time to reach its operating speed. When the clutch is first engaged, the mismatch of speeds between the engine and the PTO shaft results in a sudden and undesirable shock and may cause the engine to stall. Operators learn to repeatedly engage and disengage the clutch to effect launch, a crude approach that can result in equipment damage. A more controlled launch would be desirable.
Another problem arises in connection with driven wheel trailers. Many agricultural and military vehicles take drive to the wheels of a trailer through a power take off, helping the vehicle for example in traversing rough terrain. The speed over the ground of the trailer wheels is not always the same as that of the driven vehicle wheels. When cornering, for example, the different wheels follow paths of different radii and thus traverse the ground at different speeds. The wheels' relative speed of rotation, however, is fixed by virtue of the ratios provided by the transmission(s) driving them, no differential gear normally being provided. Consequently wheel slip is inevitable, but is accompanied by very large and undesirable power recirculation through the transmission(s). Despite the robust construction of agricultural vehicles, expensive failures do occur as a result. The problem arises not only due to cornering, but also because of mismatches in wheel size, uneven terrain etc. It would be desirable to provide some means of driving the PTO which enabled it to match its speed to that of the vehicle driven wheels, while providing the necessary trailer wheel torque.
Another problem of known PTOs concerns energy efficiency. Normally fixed drive ratios are chosen on the basis that the engine is to operate at high speed, at or near the speed at which it is capable of providing its peak power. This is so that when necessary high power can be provided to the driven implement without departing from its required drive speed. The result is that even when the power required by the implement is relatively low, the engine must be run at the same fixed high speed, which burns fuel unnecessarily.
A final difficulty is encountered when the load on the PTO varies. An example is encountered when using a forage harvester, an implement which is mounted upon the vehicle to collect cut material, and which consumes high power. When the forage harvester encounters a particularly dense region of material, the torque needed to drive it increases. The power needed to maintain PTO speed may exceed that available from the engine. In this situation it is desirable to permit the PTO to slow down, rather than stall the engine. Preferably in doing so a high engine speed should be maintained in order to provide high engine power.
It has previously been proposed to use a continuously variable transmission—that is, a transmission able to provide a stepless variation in drive ratio—to drive a PTO. Reference is directed in this regard to published international patent application US2003/0070819, application Ser. No. 10/236,589, in the name of Hrazdera. Advantages arising from the use of a CVT, including improved launch and flexibility in the speed of the driving engine, are recognised therein. Problems of control remain. For example a conventional CVT does not in itself protect the engine from stall in response to excess load created by the load
Searches conducted by the European Patent Office on the present invention contained reference to EP1106870 (Kawasaki Jukogu Kabushiki Kaisha). This describes an apparatus for driving an aircraft generator from a jet engine at constant speed. The apparatus uses a full or part-toroidal variator to regulate the generator speed, but it is clear that this is a ratio controlled device. Reference is directed in this regard to FIG. 10 in the related description at paragraph 55. The same search drew attention to U.S. Pat. No. 4,186,616 (Sharpe) which again concerns a transmission intended for driving an aircraft alternator. It uses a toroidal type variator, but whether this is torque controlled is not specified. Of course an alternator drive for an aircraft is not a power take off for a motor vehicle, in the sense in which that phrase is used herein.